Tank trucks, particularly those used for transporting inflammable liquids, such as gasoline, typically include a dump hose which is connected to a discharge conduit or manifold mounted on the bottom of the tank for unloading or loading. A manually operated valve, such as a gate valve or a lever-operated faucet, is located in the discharge conduit for shutting off flow of the liquid from or into the tank. A safety valve is located in each tank compartment at the inlet to the discharge conduit as a safety measure in the event the discharge conduit is accidentally ruptured or broken away, such as during a collision. The safety valve usually is manually opened for unloading or loading, but as a rule, is not designed to control flow. A shutoff valve is used for this purpose and the safety valve is closed after flow has been shut off by closing the shutoff valve. Consequently, a quantity of the liquid (i.e., several gallons for larger systems) is trapped in the discharge conduit between the safety valve and the shutoff valve. Tank trucks often travel with this so-called "wet line." Needless to say, this trapped liquid represents a potential fire hazard in the event the discharge conduit is accidentally ruptured during a collision.
The discharge conduit can be partially drained by opening a shutoff valve after the safety valve has been closed. However, the vacuum effect created between the safety valve and the head of the liquid column, as the liquid starts to drain, prevents the discharge conduit from being completely emptied by gravity. This vacuum effect can be eliminated by venting the discharge conduit below the safety valve. However, safety regulations require that the vapors of inflammable liquids, such as gasoline, be vented into the storage tank or the supply tank to which the tank truck is connected.